1. Field of the Invention
The present invention relates to optical lens units including lens barrels that contain lenses and methods for producing the optical lens units, and in particular, relates to optical lens units having lens surfaces shaped by heating glass bulbs in lens barrels and methods for producing the optical lens units.
2. Description of the Related Art
Typical optical lens units including glass lenses held inside metallic lens barrels are formed by placing glass bulbs serving as lens bases inside the lens barrels, heating the glass bulbs until the glass bulbs are softened, and deforming the glass bulbs by vertical clamping using molds such that the glass bulbs are bonded to the insides of the lens barrels by pressure and such that lens surfaces are formed in optical-axis directions of the lens barrels by transferring the shapes of the mold surfaces to the lens bases. Japanese Unexamined Patent Application Publication No. 2002-6189, for example, describes an optical lens unit produced in this manner.
FIG. 5 is a cross-sectional view illustrating a production process of a known optical lens unit. In FIG. 5, a glass bulb 55 is disposed inside a lens barrel 50. As shown in FIG. 5, the lens barrel 50 is hollow, and has an inner wall 51. The lens barrel 50 has an inclined surface 54 formed on the inner wall 51 between a first opening 52 and a second opening 53. With this, the diameter of the second opening 53 is smaller than that of the first opening 52. The tilting angle θ of the inclined surface 54 formed on the inner wall 51 of the lens barrel 50 ranges from 40° to 70°, and the glass bulb 55 is disposed so as to be brought into contact with an end of the inclined surface 54 adjacent to the second opening 53.
The glass bulb 55 is often preheated in the state shown in FIG. 5 in order to remove impurities on the surface thereof before the lens surfaces are shaped. In this case, the end of the inclined surface 54 adjacent to the second opening 53 digs into the glass bulb 55 that is slightly softened by heating, and a groove 56 is formed. When the glass bulb 55 is rotated while the lens barrel is carried to a forming press after preheating or by vibration during press-forming, the groove 56 formed during preheating can be located at positions where the lens surfaces are to be formed as shown in FIG. 6. In this case, the groove remains as an imprint after the lens surfaces are shaped, and the optical lens unit is rejected.